The invention relates to the field of magnetoresistive transducers (heads) and more particularly to magnetoresistive heads used in data storages systems and even more particularly to giant magnetoresistive (GMR) heads.
A typical prior art head and disk system is illustrated in FIG. 1. In operation the head 10 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer, usually called a xe2x80x9chead,xe2x80x9d is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. Typically there are two electrical contact pads (not shown) each for the read and write heads 12, 23. Wires or leads 14 are connected to these pads and routed in the suspension 13 to the arm electronics (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded. The read head 12 reads magnetic transitions as the disk rotates under the air-bearing surface of the head 10.
There are several types of read heads 12 including those using spin valves and tunnel junctions. Spin valves exhibit a much larger magnetoresistive effect than anisotropic magnetoresistive (AMR) sensors and are, therefore, referred to as giant magnetoresistive (GMR) sensors. Thus, heads using spin valves are called GMR heads.
The basic structure of a spin valve sensor (not shown) includes an antiferromagnetic layer, a pinned layer and a free layer. The spin valve effect is a result of differential switching of two weakly coupled ferromagnetic layers separated by a nonmagnetic spacer layer of, for example, copper. The antiferromagnetic layer fixes the magnetic moment of the pinned layer 90 degrees with respect to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. The quiescent position is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from a rotating magnetic disk. The quiescent position of the magnetic moment of the free layer is preferably parallel to the ABS. The magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from a moving magnetic medium.
The thickness of the spacer layer is chosen to be less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces or boundaries of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in the resistance of the spin valve sensor is a function of cosine of the angle between the magnetic moments of the pinned and free layers. This resistance is referred to in the art as magnetoresistance (MR).
In one variation the pinned layer function is performed by a multilayer structure which includes a nano-oxide layer (NOL). The free layer may also be multilayer structure which includes an NOL. Use of an NOL in the free or pinned layer structure has been reported to improve the MR ratios through a specular scattering effect of the conduction electrons. The NOLs have also been suggested for use in dual spin valves. (See H. Sakakima, et al., xe2x80x9cEnhancement of MR ratios using thin oxide layers in PtMn and xcex1-Fe2O3-based spin valvesxe2x80x9d; J. Magnetism and Magnetic Materials 210(2000) L20-24).
In U.S. Pat. No. 5,966,272 W. C. Cain describes a xe2x80x9ctrilayerxe2x80x9d structure for use in a magnetoresistive head which consists of an MR layer, a spacer layer, an exchange layer and a soft adjacent layer(SAL). The MR layer is longitudinally biased by hard magnetic materials at the ends of the MR layer. The SAL produces a transverse bias when the sense current flows through the trilayer structure. The exchange layer also produces a transverse bias which can saturate the SAL with zero current or a small current. The reduction of the sense current needed in the head of ""272 is stated to be the improvement over the prior art. The exchange layer is composed of NiO/CoO or FeMn from 15 to 30 nm thick.
In U.S. Pat. No. 6,219,208 the present applicant described the use of a dual spin valve sensor with a self-pinned layer which has its magnetic moment pinned perpendicular to an air bearing surface by sense current fields from conductive layers in the dual spin valve sensor when a sense current is conducted there through. This scheme eliminates one of the antiferromagnetic pinning layers that is typically employed in a dual spin valve sensor. The self-pinned layer is thin so that its demagnetization field will not be greater than the sense current fields acting thereon. Because of the thinning of the self-pinned layer the spin valve effect of the spin valve sensor is degraded by scattering of conduction electrons at the boundary of the self-pinned layer. In order to overcome this problem a specular reflector layer is employed in contact with the self-pinned layer for reflecting the conduction electrons back into a mean free path of conduction electrons so that the spin valve effect on the self-pinned layer side of the spin valve sensor can be added to another spin valve effect on the other side of the free layer structure for providing a double spin valve effect with an improved read gap, as compared to prior art dual spin valve sensors. Copper, gold and silver are suggested for the specular layers.
Although NOL have been used in the prior art of GMR sensors, the embodiments have suffered from the fact that the NOL material pinned a portion of the free layer and, therefore, increased the coercivity, i.e., hardened the free layer. Thus, there is a need for sensor designs which retain the benefits of the NOL while reducing the undesirable side effects.
A head according to the invention includes a free layer structure comprising two free layers exchange coupled across a thin spacer structure comprising two spacer layers of nonmagnetic material separated by a ferromagnetic nano-oxide layer (NOL). The spacer layers prevent the NOL from unnecessarily hardening the free layer(s). The spacer layers are preferably copper or copper oxide. The spacer layers preserve the NOL""s property of specular scattering of conduction electrons which tends to increase the magnetoresistive response. A free layer structure including the exchange coupling spacer structure of the invention can be used in a dual spin valve configuration, but is also useful in a single spin valve configuration. The free layer structure of the invention is useful in conduction in-plane (CIP), as well as, conduction perpendicular to the plane (CPP) devices. Many prior art CPP devices have been impractical due to their low resistance. The free layer structure of the invention provides an additional benefit for CPP devices by raising the resistance due to the resistive properties of the NOL""s.